Metal-on-ceramic substrates

ABSTRACT

A metal-on-ceramic substrate comprises a ceramic layer, a first metal layer, and a bonding layer joining the ceramic layer to the first metal layer. The bonding layer includes thermoplastic polyimide adhesive that contains thermally conductive particles. This permits the substrate to withstand most common die attach operations, reduces residual stress in the substrate, and simplifies manufacturing processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/261,424, filed on Dec. 1, 2015, the disclosure of which ishereby fully incorporated by reference herein.

BACKGROUND

The present disclosure relates to circuit fabrication. Moreparticularly, the present disclosure involves the use of aparticle-filled adhesive bonding layer which is thermally conductive inthe production of a metal-on-ceramic substrate.

Metal-on-ceramic substrates are extensively used for production ofelectronic circuits. The metal layer can be shaped, e.g. photoetched, toform conductive paths, allowing for these substrates to replacetraditional wired circuits. The metal is typically copper. The ceramicis typically alumina, beryllia (BeO), silicon nitride (Si₃N₄), oraluminum nitride (AlN). Of the four, AlN offers the highest thermalconductivity without the environmental health and safety (EH&S) concernsof BeO.

Conventional metal-on-AlN and metal-on-Si₃N₄ substrates have a number oflimitations. One such limitation is the lack of a stable metal-ceramicbond phase. Presently, thick (8 to 16 mils thick) sheets of coppercannot be directly, chemically bonded to an AlN or Si₃N₄ ceramicsubstrate. There are thus two approaches in the industry to form a thickcopper layer on an AlN ceramic substrate.

The first approach is known as direct bond copper (DBC). The surface ofthe AlN is oxidized to form a surface scale, viz. alumina on an aluminumnitride ceramic. An alumina-copper-oxide (AlCu_(x)O_(y)) then forms,which acts as a bond layer to both the alumina scale and the copper. TheDBC process typically occurs at temperatures of around 1070° C.

The second approach is known as active metal braze (AMB) or vacuumbrazing. The metal can be brazed to the ceramic using an active brazingalloy such as CuSil-ABA (commercially available from Morgan AdvancedMaterials, containing 63.0% silver, 34.25% copper, and 1.75% titanium)under vacuum. This process typically occurs at temperatures of around825° C. Active metal brazing is routinely used to bond sheets of copperto AlN and Si₃N₄ substrates.

These approaches result in residual stress between the metal and theceramic due to the severe coefficient of thermal expansion (“CTE”)mismatch between the metal and the ceramic. For example, copper has aCTE of 17.9 ppm/° C., aluminum nitride (AlN) has a CTE of 4.4 ppm/° C.,and Si₃N₄ has a CTE of 3.4 ppm/° C. The high CTE mismatch is anotherlimitation of conventional metal-on-ceramic substrates. This CTEmismatch requires both faces of the ceramic to be coated with metal ofequal thickness in order to balance the stress and minimize bowing orcracking of the ceramic. This need to have thick metal on both facesincreases production costs. In addition, the bond layer between themetal and the AlN or Si₃N₄ ceramic usually has high thermal resistance.This reduces the ability of the ceramic to absorb and dissipate heatgenerated in the metal layer, or generated by semiconductor chips bondedto the metal layer.

A third approach is to pattern metallize a ceramic using a thin filmprocess to create a “seed” layer, then up-plate the seed layer to athick metal layer by an electrolytic or electroless process. Thisprocess exhibits the lowest thermal resistance between the metal and theceramic and it produces thick layers with no stress from CTE mismatch,but this process suffers from high cost and limited gap widths betweenthe metal traces imposed by the lateral growth of the metal duringplating.

It would be desirable to provide processes for preparingmetal-on-ceramic substrates that can avoid these problems, and desirablyare of relatively lower cost and relatively simpler as well.

BRIEF DESCRIPTION

The present disclosure relates to fabrication of a metal-on-ceramicsubstrate involving a bonding layer between the metal and ceramiclayers. This substrate can be used to form electrical circuits withappropriate processing.

Disclosed in various embodiments are circuit substrates formed from afirst metal layer, a ceramic layer, and a bonding layer which comprisesa thermally conductive adhesive. Also disclosed herein are processes formaking the circuit substrate.

The bonding layer may comprise a foil layer and two adhesive layers onopposite sides of the foil layer.

In the present disclosure, the adhesive of the bonding layer may include(i) a polyimide or a thermoplastic polyimide (TPI), and (ii) thermallyconductive particles. The thermally conductive particles can be adielectric material. The thermally conductive particles may be selectedfrom the group consisting of silver, gold, copper, graphene, carbonnanotubes, hexagonal BN, würtzitic BN, cubic BN, BN nanotubes, diamond,AlN and Si₃N₄.

The first metal layer may be a metal selected from the group consistingof copper, covetic copper, copper-beryllium, aluminum, aluminum siliconcarbide (AlSiC), silver, palladium, platinum, nickel, gold, stainlesssteel, nickel-cobalt ferrous alloy, and alloys thereof. In particularembodiments, the first metal layer includes a set of sublayers. Thefirst metal layer may have a thickness of from about 1 mil to about 30mils.

The circuit substrate may, in some embodiments, further comprise asecond metal layer on a side of the ceramic layer opposite the firstmetal layer.

The ceramic layer can be made of a ceramic selected from the groupconsisting of aluminum nitride (AlN), alumina (Al₂O₃), BeO, zirconiatoughened alumina (ZTA), SiC, and Si₃N₄. The ceramic layer may have athickness of about 20 mils to about 60 mils. The ceramic may also have athree dimensional shape, such as a finned heatsink, whose totalthickness exceeds 60 mils (1.5 mm).

Also disclosed herein are processes for creating the circuit substratedescribed herein, comprising: placing the bonding layer between theceramic layer and the first metal layer; applying pressure to bond theceramic layer to the first metal layer using the bonding layer; andcuring the bonding layer.

The first metal layer can be shaped into a desired shape prior toapplying the pressure, or after curing the bonding layer.

In particular embodiments, the first metal layer is shaped by coatingthe first metal layer with photoresist, developing the photoresist toobtain a desired pattern, and removing the first metal layer with anacid in areas not covered by developed photoresist. The developedphotoresist may then be stripped in an alkaline solution. Other methodsmay be used to remove the developed photoresist, including plasmaetching or reactive ion etching.

A fixture or jig can be used to align the ceramic layer, the bondinglayer, and the first metal layer.

The applied pressure may have a magnitude of from about 65 kPa to about75 kPa. The curing can be performed by exposing the bonding layer to anelevated temperature of about 210° C. to about 290° C.

These and other non-limiting characteristics of the disclosure are moreparticularly disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for the purposes of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same.

FIG. 1 is a picture of a circuit built on a direct bond coppersubstrate.

FIG. 2 is a schematic exploded cross-section of a circuit substrateformed from a first metal layer, a bonding layer, and a ceramic layer.Here, the bonding layer is an adhesive containing thermally conductiveparticles.

FIG. 3 is a schematic exploded cross-section of a circuit substrate inwhich the bonding layer comprises a foil layer and two adhesive layerson opposite sides of the foil layer.

FIG. 4 is an illustration showing the patterning of the circuitsubstrate using photoresist.

FIG. 5 is a photo of photoetched copper on a AlN substrate. The adhesiveis diamond-filled TPI which remains as a coating over the entire topsurface of the AlN, including the gaps between the Cu pads.

DETAILED DESCRIPTION

A more complete understanding of the components, processes andapparatuses disclosed herein can be obtained by reference to theaccompanying drawings. These figures are merely schematicrepresentations based on convenience and the ease of demonstrating thepresent disclosure, and are, therefore, not intended to indicaterelative size and dimensions of the devices or components thereof and/orto define or limit the scope of the exemplary embodiments.

Although specific terms are used in the following description for thesake of clarity, these terms are intended to refer only to theparticular structure of the embodiments selected for illustration in thedrawings, and are not intended to define or limit the scope of thedisclosure. In the drawings and the following description below, it isto be understood that like numeric designations refer to components oflike function.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used in the specification and in the claims, the term “comprising”may include the embodiments “consisting of” and “consisting essentiallyof.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,”“contain(s),” and variants thereof, as used herein, are intended to beopen-ended transitional phrases, terms, or words that require thepresence of the named components/steps and permit the presence of othercomponents/steps. However, such description should be construed as alsodescribing compositions or processes as “consisting of” and “consistingessentially of” the enumerated components/steps, which allows thepresence of only the named components/steps, along with any impuritiesthat might result therefrom, and excludes other components/steps.

Numerical values in the specification and claims of this applicationshould be understood to include numerical values which are the same whenreduced to the same number of significant figures and numerical valueswhich differ from the stated value by less than the experimental errorof conventional measurement technique of the type described in thepresent application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint andindependently combinable (for example, the range of “from 2 to 10” isinclusive of the endpoints, 2 and 10, and all the intermediate values).

A value modified by a term or terms, such as “about” and“substantially,” may not be limited to the precise value specified. Themodifier “about” should also be considered as disclosing the rangedefined by the absolute values of the two endpoints. For example, theexpression “from about 2 to about 4” also discloses the range “from 2 to4.”

As used herein, the term “coefficient of thermal expansion” or “CTE”refers to the linear coefficient of thermal expansion at 20° C.

A “mil” is one-thousandths of an inch.

FIG. 1 is a picture of a circuit built on a direct bond coppersubstrate. The copper layer has been etched to form conductive pathsbetween various components that are attached to the alumina substrate.

The present disclosure relates to circuit substrates that are made froma metal layer, a bonding layer, and a ceramic layer. The bonding layerincludes an adhesive mixed with thermally conductive particles. Thebonding layer can be cured at temperatures of less than 300° C., whichis much lower than other prior art processes. However, after curing, thesubstrate can withstand extended operation at temperatures higher thanthe curing temperature. The resulting substrate also has reduced thermalstress, and the manufacturing process is lower in cost.

FIG. 2 is an exploded cross-sectional view of a metal-on-ceramicsubstrate 200 of the present disclosure. This circuit substrate isformed from a ceramic layer 210, a first metal layer 240 that is to bejoined to one side of the ceramic layer, and a bonding layer 220 locatedbetween the ceramic layer 210 and the first metal layer 240. Pressure isapplied to join the three layers together, and the substrate is thenexposed to high temperature to cause curing of the bonding layer. Thebonding layer 220 includes a thermally conductive adhesive. Inparticular embodiments, curing is performed by applying a pressure fromabout 65 kPa to about 75 kPa and a temperature from about 210° C. toabout 290° C. Curing can be performed in air, dry nitrogen or vacuum.

The ceramic layer 210 can be a ceramic selected from the groupconsisting of aluminum nitride, alumina, beryllium oxide, zirconiatoughened alumina, silicon carbide, or silicon nitride. The ceramiclayer can have a thickness of about 20 mils to about 60 mils. Inparticularly desired embodiments, the ceramic layer is aluminum nitride(AlN). The ceramic layer generally has high thermal conductivity, and isused to transport heat generated within the metal layer away from themetal layer and towards a heatsink.

The metal that forms the first metal layer 240 can be chosen fromcopper, covetic copper, copper-beryllium, aluminum,aluminum-silicon-carbide (AlSiC), silver, gold, palladium, platinum,nickel, gold, stainless steel, nickel-cobalt ferrous alloy, and alloysthereof. The metal layer can have a thickness of about 1 mil to about 30mils. In particularly desired embodiments, the first metal layer is madeof copper.

In some desirable embodiments, the first metal layer can be formed froma set of sublayers. This may be useful to protect a given sublayer withother sublayers. In such embodiments, there may be a minimum of twosublayers or of three sublayers. For example, a copper sublayer can beplated or clad on one or both faces with another metal sublayer toprevent oxidation of the copper, prior to being bonded to the ceramiclayer. As another example, a sheet of copper could be plated on bothfaces with nickel, then palladium, then gold, and then be bonded to theceramic layer. In a cross-sectional view, this would form a first metallayer made up of seven total sublayers.

The bonding layer comprises an adhesive that is thermally conductive,i.e. has low thermal resistance. More specifically, the adhesiveincludes (i) a polyimide or thermoplastic polyimide or apolyimide-and-epoxy blend, mixed with (ii) thermally conductiveparticles. Most desirably, a thermoplastic polyimide is used.Thermoplastic polyimide provides a fast-acting bond and is suitable forhigh temperature operations. However, polyimides are thermal insulatorsand do not conduct heat very well.

The particles are made of a material that is thermally conductive andcan be in the form of round grains or flakes. The material can also be adielectric in some embodiments. Non-limiting examples of such particlesinclude silver, gold, copper, graphene, carbon nanotubes, hexagonal BN,würtzitic BN, cubic BN, BN nanotubes, diamond, AlN and Si₃N₄. Thesethermally conductive particles provide a thermal conduction capabilityto the polyimide and the overall adhesive. The particles may be presentin an amount of from greater than 0 to about 80 volume percent of theadhesive, including from about 10 to about 75 volume percent. Desirably,the adhesive is thermally conductive and is also electricallyinsulating. A thermally conductive, dielectric bond layer enables theadhesive to remain fully coating the surface of the ceramic after etchdefinition of the copper layer. The dielectric TPI will be able to standoff voltage between the copper pads. The bonding layer may have athickness of about 2 micrometers (μm) to about 150 μm, or from about 8μm to about 125 μm.

The adhesive can be an A-stage adhesive, in which the polyimide isliquid and a relatively significant amount of solvent is still present.However, in more desired embodiments, the adhesive is a B-staged coatingon the metal layer and/or the ceramic layer, in which the majority ofsolvent has been previously removed and the adhesive is uncured, but canbe handled and shaped relatively easily.

The resulting metal-on-ceramic substrate can be used for high current (1to 100 amperes) power devices. The metal layer is electricallyconductive, and heat generated therefrom is conducted through thebonding layer to the ceramic layer for conduction to a heatsink.Particular applications include thermoelectric coolers, high powerinverter circuits, concentrator photovoltaic modules, high powertraction motor circuits, and high current busbars.

The use of a thermoplastic polyimide filled with thermally conductiveparticles as the adhesive in the bonding layer provides severaladvantages. First, this adhesive will bond well to most ceramic, metal,or glass surfaces without requiring pre-metallization of that surface.Second, this adhesive can cure at temperatures below 300° C. Thisreduces residual stress between the ceramic layer and the metal layerthat results from the high temperatures needed for direct bond copper(1070° C.) or active metal braze (825° C.) and the subsequent coolingdown room temperature. This low temperature cure also reduces processingcosts since lower-cost ovens or hot plates can be used instead ofexpensive high temperature furnaces.

Another advantage is that this adhesive can be operated above its curingtemperature without degrading. This permits higher operatingtemperatures in the final substrate, and also allows the circuits towithstand higher amperage excursions compared to other adhesives. Oncecured, the thermoplastic polyimide filled with thermally conductiveparticles can withstand extended operation in air at 350° C. and thermalexcursions to 450° C. By comparison, epoxy adhesives typically cure at alow temperature of around 170° C., and will debond, char or delaminateat higher temperatures. A substrate made per the current disclosure iscompatible with subsequent die attach using conventional die bondingmaterials such as silver-filled epoxy, AuSn solder (280° C.), and SnAgCusolder (217° C.).

Yet another advantage that arises is that due to the reduced residualstress, new metal layers can be bonded to the ceramic substrate thanwere previously possible. For example, an aluminum-clad copper sheet canbe used as the metal layer. The metal layer can be of variable thicknessand topography/shape. For example, the metal layer could include aterminal post, an LED reflector, holes or slots, heatsink fins, or otherfeatures that could be stamped, milled, or photoetched into the metallayer. Put another way, the metal layer can now be shaped into variousthree-dimensional forms.

For substrates made per the prior art, severe residual stress due to CTEmismatch previously required metal layers to be placed on bothsides/faces of the ceramic layer to prevent bowing. In the presentdisclosure, the thickness of metal layers on opposite sides of theceramic layer can be significantly different, while the ceramic layermaintains its flatness after the bonding layer/adhesive is cured.Indeed, under many combinations of metal and ceramic, a metal layer onlyneeds to be present on one side of the ceramic layer when the bondinglayer of the present disclosure is used. This can be advantageous incertain applications, plus it can reduce cost.

Next, the metal layer can be electroplated or clad prior to bonding. Forexample, a copper sheet can be electroplated with nickel, palladium,gold, silver or combinations thereof. The copper sheet could be cladwith aluminum, stainless steel, or combinations thereof. At the higherprocessing temperatures of the prior art needed to bond the ceramic tothe metal layer, these plated/clad layers would diffuse into the copperand lose their desirable properties. With the reduced bondingtemperatures of 300° C. or less of the present disclosure, thesesublayers can be maintained.

FIG. 3 is a cross-sectional view of a second exemplary embodiment of ametal-on-ceramic substrate 300 of the present disclosure, illustratingsome variations and differences in the layers that are contemplated bythe present disclosure.

This embodiment includes a ceramic layer 310, a bonding layer 320, and ametal layer 340. The bonding layer is comprised of a metal foil 322 andtwo adhesive layers 324, located on opposite sides of the foil 322. Theadhesive layers 324 are formed of (i) a polyimide or thermoplasticpolyimide or a polyimide+epoxy blend, mixed with (ii) thermallyconductive particles, as described above. The metal layer 340 here isformed from a set of sublayers. A copper sheet is plated with nickel,then palladium, then flash gold, on top of the copper sheet, for a totalof four sublayers 342, 344, 346, 348.

Other embodiments of the metal-on-ceramic substrate are alsocontemplated, though not illustrated. In FIG. 2 and FIG. 3, only a firstmetal layer is present on one side of the ceramic layer. It is alsocontemplated that a second metal layer could be placed on the oppositeside of the ceramic layer, so that the ceramic layer is between the twometal layers. A bonding layer using the adhesive of the presentdisclosure (thermoplastic polyimide plus thermally conductive particles)can also be used to bond the second metal layer to the ceramic layer.The first metal layer and the second metal layer can have differentthicknesses from each other.

It is noted that the metal layer(s) of the ceramic-on-metal substratecan be processed to obtain a desired shape. This processing can be doneprior to bonding the metal layer to the ceramic layer, or can be doneafter bonding to the ceramic layer. In some embodiments, the first metallayer 240, 340 and the bonding layer 220, 320 may be machined into thedesired shape prior to bonding. For example, the bonding layer may beaffixed to the metal layer, and the two layers can then be stamped orcut into the desired pattern. Alternatively, the bonding layer and themetal layer can be stamped or cut into the desired pattern separately. Afixture or jig can then be used to align the ceramic layer, thepatterned bonding layer, and the patterned metal layer. Pressure isapplied to join the three layers together, then the adhesive is cured.

In other embodiments, the substrate is first formed by bonding thelayers together and curing the adhesive. Subsequently, the first metallayer and the bonding layer are photoetched to obtain the desiredpattern.

In this regard, the thermoplastic polyimide used in the adhesive, oncefully cured, will resist etching by acids and bases.

FIG. 4 is an illustration of one method for post-etching the metallayer. The substrate starts at the left (labeled A) with the ceramiclayer 410, bonding layer 420, and first metal layer 440 already bondedtogether. Next, photoresist 450 is applied as a layer upon the firstmetal layer, and then exposed to obtain the desired pattern (labeled B).Next, acid is used to etch the first metal layer (labeled C). Suitableacids that etch copper include ferric chloride and cupric chloride.Next, an alkaline solution is used to strip the developed photoresist(labeled D).

FIG. 5 is a photo of a substrate fabricated according to embodiments ofthe present disclosure. A metal layer of 10 mil thick Cu was bonded toan AlN substrate using TPI filled with diamond powder. The Cu wasphotoetched to define isolated pads. None of the chemicals used forphotoetching corroded the TPI bonding layer. The diamond-filled TPIbonding layer is a thermally conductive dielectric that provides goodelectrical isolation between the Cu pads.

The following examples are provided to illustrate the substrates andprocesses of the present disclosure. The examples are merelyillustrative and are not intended to limit the disclosure to thematerials, conditions, or process parameters set forth therein.

EXAMPLES Example 1

A copper sheet is provided that has dimensions of 5 inches×7inches×0.012 inches thickness. The 0.012″ thick copper sheet is coatedon one face with an A-stage adhesive formed from thermoplastic polyimide(TPI) that is filled with diamond particles having a diameter of 1-2microns. Spin coating is used to deposit a layer of A-staged,diamond-filled TPI uniformly over the area of the copper sheet. The TPIlayer is then B-staged to a thickness of 3-5 μm. Next, an unmetallizedsheet of aluminum nitride of dimensions 5 inches×7 inches×0.020 inchesthickness is positioned beneath the TPI-coated copper sheet. The copperand bonding layer are both aligned with the ceramic substrate. Apressure of about 10 psi (69 kPa) is applied and the TPI is cured at280° C. in a vacuum oven.

The copper face is then coated with photoresist, the photoresist ispatterned, and the copper is etched in an acid such as ferric chloride.The developed photoresist is then stripped in an alkaline solution so asto remove all of the developed photoresist. The alkaline solution isrinsed away. The copper is then thoroughly cleaned of residual resistand is then plated with nickel, palladium, and flash gold.

Example 2

A copper sheet of dimensions 5 inches×7 inches×0.012 inches thickness isphotoetched to the desired shape. The copper sheet is then electroplatedwith nickel, palladium, and flash gold on all surfaces (i.e. the coppersheet is a sublayer encased or encapsulated with other metal sublayers).A bonding layer is made of 3 μm of silver-filled, B-staged thermoplasticpolyimide on both sides of a 25 μm foil made of silver. The bondinglayer is then stamped into the same dimensions and shape as thephotoetched copper. Then, an unmetalized sheet of aluminum nitride ofdimensions 5 inches×7 inches×0.020 inches is positioned beneath thebonding layer and the copper sheet. The copper sheet and bonding layerare both aligned atop the aluminum nitride sheet using a fixture or jig.A pressure of about 10 psi (69 kPa) is applied to join the three layerstogether, and the bond film is cured at 280° C. to obtain the finishedsubstrate.

Example 3

A copper sheet is provided that has dimensions of 5 inches×7inches×0.012 inches thickness. The copper sheet is coated on one facewith an A-stage adhesive formed from thermoplastic polyimide (TPI) thatis filled with silver flakes. The TPI layer is then B-staged to athickness of 3 μm. The sheet is then formed to the desired shape bymachining, wire EDM, waterjet cutting, or stamping in such a way as tonot harm the TPI layers bonded to the copper. Next, an unmetallizedsheet of aluminum nitride of dimensions 5 inches×7 inches×0.020 inchesthickness is positioned beneath the TPI-coated copper sheet. The copperand bonding layer are both aligned with the ceramic substrate. Apressure of about 10 psi (69 kPa) is applied and the TPI is cured at280° C.

The copper is then thoroughly cleaned in solutions and is then platedwith nickel, palladium, and flash gold.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

The invention claimed is:
 1. A circuit substrate comprising: a ceramiclayer; a first metal layer; and a bonding layer joining the first metallayer to the ceramic layer, the bonding layer comprising an adhesivewhich is thermally conductive; and a second metal layer on a side of theceramic layer opposite the first metal layer; wherein the adhesive ofthe bonding layer consists of (i) a thermoplastic polyimide, and (ii)thermally conductive particles; and wherein the thermally conductiveparticles are selected from the group consisting of silver, copper,gold, diamond, and Si₃N₄.
 2. The circuit substrate of claim 1, whereinthe bonding layer comprises a foil layer and two adhesive layers onopposite sides of the foil layer.
 3. The circuit substrate of claim 1,wherein the thermally conductive particles are selected from the groupconsisting of silver, copper, and gold.
 4. The circuit substrate ofclaim 1, wherein the first metal layer is a metal selected from thegroup consisting of copper, covetic copper, copper-beryllium, aluminum,aluminum silicon carbide (AlSiC), silver, palladium, platinum, nickel,gold, stainless steel, nickel-cobalt ferrous alloy, and alloys thereof.5. The circuit substrate of claim 4, wherein the first metal layerincludes a set of sublayers.
 6. The circuit substrate of claim 1,wherein the first metal layer has a thickness of from about 1 mil toabout 30 mils.
 7. The circuit substrate of claim 1, wherein the ceramiclayer is made of a ceramic selected from the group consisting ofaluminum nitride, alumina, BeO, zirconia toughened alumina (ZTA), SiC,and Si₃N₄.
 8. The circuit substrate of claim 1, wherein the ceramiclayer has a thickness of about 20 mils to about 60 mils.
 9. The circuitsubstrate of claim 1, wherein the thermally conductive particles areround or flake.
 10. A circuit substrate comprising: a ceramic layer; afirst metal layer; and a bonding layer joining the first metal layer tothe ceramic layer, the bonding layer comprising an adhesive which isthermally conductive; and a second metal layer on a side of the ceramiclayer opposite the first metal layer; wherein the adhesive of thebonding layer consists of (i) a thermoplastic polyimide, and (ii)thermally conductive particles, and wherein the bonding layer is curedat a temperature from 210° C. to 290° C.
 11. The circuit substrate ofclaim 10, wherein the bonding layer is thermally stable up to 350° C. inair.
 12. The circuit substrate of claim 10, wherein the bonding layer iscured at a pressure from 65 kPa to 75 kPa, and wherein the bonding layeris cured in air, dry nitrogen, or vacuum.
 13. The circuit substrate ofclaim 10, wherein the thermally conductive particles are selected fromthe group consisting of silver, copper, gold, graphene, carbonnanotubes, diamond, boron nitride (BN), aluminum nitride (AlN), andsilicon nitride (Si₃N₄).
 14. The circuit substrate of claim 10, whereinthe thermally conductive particles are selected from the groupconsisting of silver, copper, gold, diamond, and Si₃N₄.
 15. The circuitsubstrate of claim 10, wherein the first metal layer is a metal selectedfrom the group consisting of copper, covetic copper, copper-beryllium,aluminum, aluminum silicon carbide (AlSiC), silver, palladium, platinum,nickel, gold, stainless steel, nickel-cobalt ferrous alloy, and alloysthereof.
 16. The circuit substrate of claim 10, wherein the ceramiclayer is made of a ceramic selected from the group consisting ofaluminum nitride, alumina, BeO, zirconia toughened alumina (ZTA), SiC,and Si₃N₄.
 17. The circuit substrate of claim 10, wherein the bondinglayer comprises a foil layer and two adhesive layers on opposite sidesof the foil layer.
 18. The circuit substrate of claim 17, wherein thefoil layer is silver and the first metal layer is a metal selected fromthe group consisting of copper, covetic copper, copper-beryllium,aluminum, aluminum silicon carbide (AlSiC), palladium, platinum, nickel,gold, stainless steel, nickel-cobalt ferrous alloy, and alloys thereof.19. The circuit substrate of claim 10, wherein the bonding layer has athickness of from about 2 μm to about 150 μm.
 20. The circuit substrateof claim 19, wherein the bonding layer has a thickness of from about 8μm to about 125 μm.